Electrostatographic developer composition

ABSTRACT

Positive and negative copies are formed from both positive and negative originals by developing with both positively and negatively charged toner and selectively transferring toner of one polarity to produce a positive or negative print. The developer may be comprised of two toners and a single carrier; two carriers and a single toner or a single toner and a single carrier.

United States Patent Lipani 51 Dec. 16, 1975 I 1 ELECTROSTATOGRAPHICDEVELOPER 2,940,934 6/1960 Carlson 252/621 COMPOSITION 3,236,776 2/1966Tomanek 252/621 3,533,835 10/1970 Hagenbach et a1, 252/621 Anthony F.Lipani, Webster, NY.

Xerox Corporation, Stamford, Conn.

Filed: July 24, 1972 Appl. No.: 274,394

Related US. Application Data Inventor:

Assignee:

Continuation-in-part of Ser. No. 214,491, Dec. 30,

1971, abandoned.

US. Cl. 252/621; 96/1 SD; 96/1.4;

427/20; 427/24; 427/25 Int. CL G03G 9/00; (303G 13/14; G036 9/02 Fieldof Search 252/621, 96/1.4

References Cited UNITED STATES PATENTS 6/1959 Giaimo, Jr. 252/621 OTHERPUBLICATIONS Hersh et al., Textile Research Journal, Vol. XXVI, No. 12,Dec. 1956, pp. 903-913.

Primary Examiner-Jack P. Brammer Attorney, Agent, or Firm-James J.Ralabate; Donald C. Kolasch; Ernest F. Chapman [5 7 1 ABSTRACT 9 Claims,No Drawings ELECTROSTATOGRAPHIC DEVELOPER COMPOSITION This applicationis a continuation-in-part of application Ser. No. 214,491 filed Dec. 30,1971 now abandoned.

This invention relates to electrostatography, and more particularly toimproved electrostatographic developing materials and the use thereof.

Electrostatography is best exemplified by electrophotography. The basicelectrophotographic process, as taught by C. F. Carlson in US. Pat. No.2,297,691, involves placing a uniform electrostatic charge on aphotoconductive insulating layer, exposing the layer to alight-and-shadow image to dissipate the charge on the areas of the layerexposed to the light and developing the resulting latent electrostaticimage by depositing on the image a finely divided electroscopic materialreferred to in the art as toner". The toner will normally be attractedto those areas of the layer which retain a charge, thereby forming atoner image corresponding to the latent electrostatic image. This powderimage may then be transferred to a support surface such as paper. Thetransferred image may subsequently be permanently affixed to the supportsurface as by heat. Instead of latent image formation by uniformlycharging the photoconductive layer and then exposing the layer to alight-and-shadow image, one may form the latent image by directlycharging the layer in image configuration. The powder image may be fixedto the photoconductive layer if elimination of the powder image transferstep is desired. Other suitable fixing means such as solvent orovercoating treatment may be substituted for the foregoing heat fixingsteps.

The electrophotographic processes generally involve positive to positivedevelopment; i.e., the production of a positive print from a positiveoriginal. In some cases, however, negative to positive development isrequired; i.e., the production of a positive print from a negativeoriginal.

The production of positive prints from negative originals generallyinvolves a change in the developing material. In accordance with thistechnique, the electrostatographic recording surface is positivelycharged and exposed to the negative original, resulting in a dischargeof the image areas, with a negative charge being induced on the surfaceof the recording surface near the edges of the image area as a result ofthe fringing field effect. The use of a positively charged toner resultsin a deposition of the toner in the negatively charged areas, resultingin the development of a positive image on the recording surface, whichcan then be transferred as known in the art. In the case of a negativelycharged recording surface, the use of a negatively charged toner willresult in the development of a positive image.

The production of positive prints from solid area in the non-dischargedareas, and in the image areas where the positive charge on the recordingsurface is zero, no neutralization occurs, and a negative charge isinduced on such areas. The use of a positive toner results in thedevelopment of a positive image on the recording surface. In the case ofa negatively charged recording surface, a negative potential is appliedto the development electrode, and the induced positive charge, in theimage areas, is developed with a negative toner to produce a positiveimage on the recording surface.

While the above techniques are capable of producing positive copies fromboth positive and negative originals, such techniques suffer fromcertain deficiencies; in particular the production of a machine which iscapable of being easily used for producing, for example, positive printsfrom both positive and negative originals. In accordance with the priorart, a user employed either two different machines or a single machinewith the capability of changing the toner in order to be able to providepositive prints from both positive and negative originals. The use oftwo machines entails higher costs and the changing of toner requires theexpenditure of unnecessary time. Accordingly, there is a need for adevelopment technique which is capable of producing both positive andnegative copies from both positive and negative originals.

An object of the present invention is to provide for improvedelectrostatographic development.

Another object of the present invention is to provide forelectrostatographic development capable of producing both negative andpositive copies from both negative and positive originals.

A further object of this invention is to provide for electrostatographicdevelopment capable of producing both negative and positive copies fromboth negative and positive originals without changing developer.

These and other objects of the present invention should be apparent tothose skilled in the art from the teachings herein.

The objects of this invention are broadly accomplished, in one aspect,by forming an electrostatographic latent image corresponding to apositive or negative original and subsequently developing the latentimage with both positively and negatively charged toner whereby toner ofone polarity is deposited in the image area and toner of the otherpolarity is deposited in at least the background area immediatelyadjacent the image area. A positive or negative print of the original isthen produced by transferring toner of one polarity to a receivingsurface.

In accordance with the present invention, the positive and negativetoner used in the development may be provided by the use of: twodifferent toners and a single carrier with the two toners being selectedwith respect to their triboelectric properties so that one of the tonersis above and the other below the carrier material in the triboelectricseries whereby the carrier material imparts a positive charge to onetoner and a negative charge to the other; a single toner and twodifferent carriers with at least the surface of the two carriers beingformed of materials selected with respect to their triboelectricproperties so that one of the carriers is above and the other below thetoner in the triboelectric series whereby one carrier imparts a negativecharge to the toner and the other carrier a positive charge to thetoner; or a single toner and a single carrier with at least the surfaceof the carrier being formed of a material 3 which is capable ofimparting both a negative and a positive charge to a single toner.

In accordance with a preferred embodiment of the present inventiondevelopment is effected by the use of a developer of a single tonerelectrostatically coated on a single carrier, the toner being bothpositively and negatively charged. In this manner, a single developermay be employed to provide both electropositive and electronegativetoner.

In accordance with the present invention, at least the surface of anelectrostatographic developer carrier is formed of a material which iscapable of imparting both a negative and a positive charge to a singletoner. The carrier may have at least its surface formed from either asingle component, as in the case of an electret, as hereinafterdescribed, or from two or more components, with such two or morecomponents being selected with respect to their triboelectric propertiesso that one of the materials is above, and the other below, a singletoner in the triboelectric series, whereby the single toner istriboelectrically positively and negatively charged by the carriersurface.

In producing a single carrier from two or more materialsgsuch materialsare intimately mixed with each other (the two materials areincompatible; i.e., the materials do not produce solid solutions) in amanner such that the domain size of each of the components is at leastequal to the average particle size of the toner to be employed, wherebyeach of the components is capable of imparting the desired charge to thetoner. It is to be understood that the domain size of each of thecomponents could be larger than the average particle size of the toner,and in some cases, slightly smaller than the average particle size ofthe toner, provided the domain size is of a size sufficient to impartthe desired charge to the single toner.

The intimate mixture of the two or more components may be formed byintimately mixing the components; e.g., by milling, as known in the art.Alternately, one of the components may be formed with pores and vacuumimpregnated or otherwise filled with the other component. It is to beunderstood that the domain size ofeach of the various components may becontrolled in such techniques by selection of the particle size of thecomponents and the pore size of the porous material, respectively. Theprecise method of producing an intimate mixture of the at least twocomponents is deemed to be within the scope of those skilled in the artfrom the teachings herein and no further explanation is deemed necessaryfor a complete understanding of the present invention.

The carrier surface thereof could also be comprised of a singlecomponent; e.g., an electret; an electret is a material which willsustain the coexistence of segregated positive and negative chargeswhich are in close proximity to each other. As should be apparent, anelectret, as a result of the coexistence of segregated positive andnegative charges, is capable of charging a single toner bothelectropositively and electronegatively. Electrets are disclosed in moredetail in the following articles; F. Gutman, Review of Modern Physics20, pp. 457472, (July 1948); Turek, Direct Current, pp. 204-211, (Dec.1959); and Wiseman et al, Electrical Engineering, pp. 869-872, (Oct.1953). An electret particularly preferred for the purposes of thepresent invention is one formed from calcium titanate. In view of thefact that electrets and the properties 'thereof are known in the art, nofurther discussion 4 thereof is deemed necessary for a fullunderstanding of the invention.

Alternatively, as hereinabove described, the developer of the presentinvention could becomprised of two carriers and a single toner with atleast the surface of each of the carriers being formed of two differentcomponents, one of which is capable of imparting a positive charge tothe toner and the other of which is capable of imparting a negativecharge to the toner. As a further alternative two different toners maybe employed with a single carrierwith one toner being capable of beingcharged negatively by the carrier and the other toner being capable ofbeing charged positively by the carrier. The materials which are to beemployed for producing the developers of the present invention may bereadily selected by referring to any of the tribeelectric series knownin the art. As representative examples of such triboelectric series,there may be mentioned the following: Shaw and Levy, E. W. L. Proc. ofRoyal Society, 138, p. 506 (1932); The Smithsonian series, published inSmithsonian Physical Tables, p. 375, 9th Rev. Ed. 1954); and Hersh andMontgomery Textile Research Laboratories, 28, p. 903 (1956).

As representative examples of materials which are generallyelectropositive (these materials impart a negative charge to the toner),there may be mentioned: glass, metal oxides, such as calcium oxide,titanium oxide, iron oxide, calcium titanate, etc.; metals, such aslead, nickel, copper, etc.; ferrite; polymers such as ethyl cellulose,acrylic polymers, acrylate polymers, etc.; and the like. Asrepresentative examples of materials which are generally electronegative(these materials impart a positive charge to the toner), there may bementioned: metals such as selenium; polymers such as halopolymers; e.g.,polymers of tetrafluoroethylene, polymers of vinyl chloride, polymers ofvinyl fluoride, polymers of vinylidene fluoride, polymers ofchlorotrifluoroethylene; butyl rubber; and the like. The selection ofthe required materials is deemed to be within the scope of those skilledin the art from the teachings herein.

The concentration of toner (ratio of toner to carrier) and the relativeproportions of electropositive and electronegative material used in twodifferent carriers or in a single carrier, in accordance with thepresent invention, has been found to affect print quality. Thus, forexample, at increasing toner conentrations, background and imagedensities increase, and in addition, the Halo effect (the halo is aconcentration of background in the area immediately around the images)increases. The selection of the optimum toner concentration and theoptimum electropositive carrier materials to electronegative carriermaterial ratio is governed by the particular toners and carriers used.The selection of optimum concentrations and proportions may be easilyaccomplished by a series of tests. In most cases, a toner concentrationof from about 0.9 to 1.2 weight percent provides the best results. Inusing two different carrier material components either in a singlecarrier or in two different carriers, in general, best results areachieved with a weight ratio of electropositive to electronegative offrom about 111.2 to about ers is deemed to be within the scope of thoseskilled in the art. It is to be understood, however, that the teachingsof the present invention are not limited to such carriers. Thus, forexample, development can be effected by fur brush development by using abrush in which: the fibers are of the same material and two differenttoners are employed one of which is positively charged by the brushmaterial and the other of which is negatively charged by the brushmaterial; some of the fibers are formed of one material which is capableof charging a single toner positively and some of the fibers are formedof another material which is capable of charging the single tonernegatively; or the fibers may be formed of a composite of materials oneof which positively charges the toner and the other of which negativelycharges the toner. It is also to be understood that a cascadedevelopment type of developer may be employed to develop images, asknown in the art, by techniques other than cascade development; e.g.,magnetic brush development, fluidized development, etc.

The toners used in the present invention may be any one of the widevariety of pigmented or dyed resins used in the art, with the resingenerally containing a dye or pigment in a quantity up to about byweight, and particularly from about 1 to about 20% by weight, of thetoner.

Typical toner materials include: gum copal, gum sandarac, rosin,cumaroneindene resin, asphaltum, gilsonite, phenol-formaldehyde resins,rosin-modified phenol-formaldehyde resins, methacrylic resins,polystyrene resins, polypropylene resins, epoxy resins, polyethyleneresins, polyamide resins, and mixtures thereof. The particular tonermaterial to be employed obviously depends upon the separation of thetoner particles from the carrier in the triboelectric series. Among thepatents describing electroscopic toner compositions are U.S. Pat. No.2,659,670 to Copley; U.S. Pat. No. 2,753,308 to Landrigan; U.S. Pat. No.3,079,342 to Insalaco; U.S. Pat. No. 25,136 to Carlson and U.S. Pat. No.2,788,288 to Rheinfrank et al. These toners generally have an averageparticle diameter between about 1 and about microns.

The colored toner may be prepared by any one of a wide variety ofprocedures for forming a uniform dispersion of the dye or pigment in theresinous material. Thus, for example, the resinous material and asuitable pigment may be heated and blended on a rubber mill and thenallowed to cool and harden to encase the pigment within the resinousmaterial. The pigmented or dyed resinous material is then micronized;e.g., in a jet pulverizer, to particles having a particle size generallyemployed for a toner. Alternatively, the finely divided toner may beprepared by spray drying a toner composition of the colorant and resindissolved in a solvent.

The above procedures and other procedures for producing colored toner ofthe desired particle size are generally known in the art and may beemployed for producing the toner of the present invention and,therefore, no detailed discussion thereof is necessary for a fullunderstanding of the invention.

The toner may also include other materials generally employed formodifying the characteristics of a toner, such as conductive materialsto modify the triboelectric properties thereof, magnetic materials tomodify the physical properties of the toner or the like, and the use ofsuch materials is deemed to be within the scope of those skilled in theart from the teachings herein.

The developers of the present invention, as hereinabove noted, arepreferably of the type used in the cascade development technique, asmore fully described in U.S. Pat. No. 2,618,551 to Walkup, US. Pat. No.2,618,552 to Wise and U.S. Pat. No. 2,638,416 to Walkup et al. In thecascade development technique, the carrier particles are larger than thetoner particles by at least one order of magnitude of size and areshaped to roll across the latent image-bearing surface. In general, thecarrier particles should be of sufficient size so that theirgravitational or momentum force is greater than the force of attractionof the toner particles in the area of the image-bearing surface wherethe toner particles are retained, whereby the carrier will not beretained by the toner particles which are attracted to the image-bearingsurface. The carrier particles generally have a particle size from about30 to about 1000 microns, but it is to be understood, that the carrierparticles may be of a size other than as particularly described,provided that the carrier flows easily over the image-bearing surface,without requiring special means for effecting removal of the carrierparticles from the image-bearing surface.

The cascade development type of developer, comprised of toner andcarrier, as hereinabove described, may also be used for developingelectrostatic latent images by other techniques, such as magnetic brushdevelopment and fluidized development, and such uses are within thespirit and scope of the present invention.

The developers of the present invention may be used for producingpositive or negative copies from positive or negative originals.

Thus, for example, in the case of a positive line original, theelectrostatographic recording surface may be positively charged andexposed to the positive original, resulting in dissipation of the chargein the non-image area and retention of the charge in the image area,with a negative charge being induced around the edges of the image areaas a result of the fringing field effect. In employing a developer ofthe present invention, the electropositive portion of the toner isdeposited in the fringe areas around the image, and the electronegativeportion of the toner is deposited in the image area, whereby both apositive and negative image is produced on the recording surface; i.e.,the negative toner portion defines a positive image on the recordingsurface and the positive toner portion defines a negative image on therecording surface. If a positive reproduction of the positive originalis desired, transfer is effected by the use of a positive charge wherebyonly the negative toner portion, defining the positive image, istransferred to the support surface. If a negative reproduction of thepositive original is desired, transfer is effected by the use of anegative charge, whereby only the positive toner portion, defining thenegative image, is transferred to the support surface. Similarly, if therecording surface is originally charged negatively, there will be anegative charge in the image areas and an induced positive charge in thefringe areas. In this case, the negative portion of the toner definesthe negative image, and the positive portion of the toner defines thepositive image, and either a positive or negative copy may be producedby appropriate selection of the polarity of the charge used fortransferring of the toner to the support surface.

In the case of a negative line original, the electrostatographicrecording surface may be positively charged and exposed to the negativeoriginal, resulting in dissipation of the charge in the image area, andretention of the charge in the non-image area, with a negative chargebeing induced around the edges of the image area as a result of thefringing field effect. In employing a developer of the presentinvention, the electropositive portion of the toner is deposited in theeimage areas and the electronegative portion of the toner is deposited inthe image areas and the electronegative portion of the toner isdeposited in the fringe area around the image, whereby both a positiveand negative image is produced on the recording surface; i.e., thenegative toner portion defines a negative image on the recording surfaceand the positive toner portion defines a positive image on the recordingsurface. If a positive reproduction of the negative original is desired,transfer is effected by the use of a negative charge, whereby only thepositive toner portion, defining the positive image, is transferred tothe support surface. If a negative reproduction of the negative originalis desired, transfer is effected by the use of a positive charge,whereby only the negative toner portion, defining the negative image, istransferred to the support surface Similarly, if the recording surfaceis originally charged negatively, there will be a negative charge in thenon-image areas and an induced positive charge around the fringe of theimage areas. In this case, the positive portion of the toner defines thenegative image, and the negative portion of the toner defines thepositive images, and either a positive or negative copy may be producedby appropriate selection of the polarity of the charge used fortransferring of the toner to the support surface.

In the case of solid area originals, both positive and negative copiesmay be produced from both positive and negative solid area originals, ashereinabove described, except that as known in the art, a developmentelectrode is employed to increase the field above the solid areas.

The present invention is further described with respect to the followingexamples. In these examples, unless otherwise specified, all parts andproportions are by weight.

EXAMPLE I A solid composite of barium ferrite in butyl rubber,containing 60% barium ferrite, is broken into small pieces (size 40mesh) and used as an electrostatographic development carrier with atoner comprising styrene-n-butyl methacrylate copolymer, polyvinylbutyral and carbon black produced as disclosed in Example I of US. Pat.No. 3,079,324 to Insalaco, the developer composition containing 1%, byweight, of the toner.

The developer is used to develop an electrostatic latent image formedfrom a positive original on a flat selenium plate charged to +700 voltsby cascading the developer (three times) over the plate.

The image is transferred to paper using +700 volts and a positive copyof the positive original is produced.

The above is repeated, except that the image was transferred using 700volts, and a negative copy of the positive original is produced.

The above is repeated except that a negative original is used and upontransfer with +700 volts a negativelooking copy i.e., light print ondark background, of the negative original is produced. Transfer effectedwith 700 volts produces a positive-looking, i.e., dark print on lightbackground copy of the negative original.

EXAMPLE n Selenium is melted and calcium titanate having an averageparticle size of 15 microns is uniformly dispersed in the selenium toprovide a composite having 40%, by weight, calcium titanate. Thecomposite is formed into particles (size 20-40 mesh) for use as anelectrostatographic development carrier and combined with 1%, by weight,of the toner described in Example The developer is used to develop anelectrostatic latent image formed from a positive original on a flatselenium plate charged to +700 volts by cascading the developer (threetimes) over the plate.

The image is transferred to paper using +700 volts and a positive copyof the positive original is produced.

The above is repeated, except that the image was transferred using 700volts, and a negative copy of the positive original is produced.

The above is repeated except that a negative original is used and upontransfer with +700 volts a negative copy of the negative original isproduced. Transfer effected with 700 volts produces a positive copy ofthe negative original.

EXAMPLE III A developer carrier of 250 micron steel beads coated with apolymer mixture of vinyl chloride vinyl acetate polymer andstyrenemethyl methacrylate vinyl triethoxy silane polymer (1:1 weightratio of polymers) is combined with 2%, by weight, of the tonerdescribed in Example I.

The developer is used to develop an electrostatic latent image formedfrom a positive original on a flat selenium plate charged to +700 voltsby cascading the developer (three times) over the plate.

The image is transferred to paper using +700 volts and a positive copyof the positive original is produced.

The above is repeated, except that the image was transferred using 700volts, and a negative copy of the positive original is produced.

The above is repeated, except that a negative original is used and upontransfer with +700 volts a negative copy of the negative original isproduced. Transfer effected with 700 volts produces a positive copy ofthe negative original.

EXAMPLE IV An electret of calcium titanate having segregated positiveand negative charges is combined with 2%, by weight, of the tonerdescribed in Example I.

The developer is used to develop an electrostatic latent image formedfrom a positive original on a flat selenium plate charged to +700 voltsby cascading the developer (three times) over the plate.

The image is transferred to paper using +700 volts and a positive copyof the positiveoriginal is produced.

The above is repeated, except that the image was transferred using 700volts, and a negative copy of the positive original is produced.

The above is repeated except that a negative original is used and upontransfer with +700 volts a negative copy of the negative original isproduced. Transfer effected with 7OO volts produces a positive copy ofthe negative original.

EXAMPLE V A developer carrier (-40 mesh) of polytetrafluroethylenehaving dispersed therein copper metal (particle size of greater than 20microns) in a weight ratio of about 1:1 is combined with 1%, by weight,of the toner of Example I.

The developer is used to develop an electrostatic latent image formedfrom a positive original on a flat selenium plate charged to +700 voltsby cascading the developer (three times) over the plate.

The image is transferred to paper using +700 volts and a positive copyof the positive original is produced.

The above is repeated, except that the image was transferred using 700volts, and a negative copy of the positive original is produced.

The above is repeated except that a negative original is used and upontransfer with +700 volts a negative copy of the negative original isproduced. Transfer effected with 700 volts produces a positive copy ofthe negative original.

EXAMPLE VI A developer carrier of 250 micron. sand coated with a mixtureof ethyl cellulose and poly vinyl fluoride (1:1 weight ratio) iscombined with 2%, by weight, of the toner described in Example I.

The developer is used to develop an electrostatic latent image formedfrom a positive original on a flat selenium plate charged to +700 voltsby cascading the developer (three times) over the plate.

The image is transferred to paper using +700 volts and a positive copyof the positive original is produced.

The above is repeated, except that the image was transferred using 700volts, and a negative copy of the positive original is produced.

The above is repeated except that a negative original is used and upontransfer with +700 volts 3 negative copy of the negative original isproduced. Transfer effected with -700 volts produces a positive copy ofthe negative original.

EXAMPLE VII A developer carrier (20-40 mesh) of polytetrafluoroethylenehaving dispersed therein 40%, by weight, of titanium dioxide (particlesize of greater than 20 microns) is combined with 1%, by weight, of thetoner of Example I.

The developer is used to develop an electrostatic latent image formedfrom a positive original on a flat selenium plate charged to +700 voltsby cascading the developer (three times) over the plate.

The image is transferred to paper using +700 volts and a positive copyof the positive original is produced.

The above is repeated, except that the image was transferred using 700volts, and a negative copy of the positive original is produced.

The above is repeated except that a negative original is used and upontransfer with +700 volts a negative copy of the negative original isproduced. Transfer effected with 700 volts produces a positive copy ofthe negative original.

EXAMPLE VIII A developer carrier (20-40 mesh) of polyvinyl chloridehaving dispersed therein 40%, by weight, of barium ferrite (particlesize of greater than 20 micron) is 10 combined with 1%, by weight, ofthe toner of Example I.

The developer is used to develop an electrostatic latent image formedfrom a positive original on a flat selenium plate charged to +700 voltsby cascading the developer (three times) over the plate.

The image is transferred to paper using +700 volts and a positive copyof the positive original is produced.

The above is repeated, except that the image was transferred using 700volts, and a negative copy of the positive original is produced.

The above is repeated except that a negative original is used and upontransfer with +700 volts a negative copy of the negative original isproduced. Transfer effected with 700 volts produces a positive copy ofthe negative original.

EXAMPLE IX A developer carrier (20-40 mesh) of selenium having dispersedtherein 40%, by weight, of iron (particle size of greater than 20microns) is combined with 2%, by weight, of the toner of Example I.

The developer is used to develop an electrostatic latent image formedfrom a positive original on a flat selenium plate charged to +700 voltsby cascading the developer (three times) over the plate.

The image is transferred to paper using +700 volts and a positive copyof the positive original is produced.

The above is repeated, except that the image was transferred using 700volts, and a negative copy of the positive original is produced.

The above is repeated except that a negative original is used and upontransfer with +700 volts :1 negative copy of the negative original isproduced. Transfer effected with 700 volts produces a positive copy.

EXAMPLE X Examples I- IX are repeated with a toner comprising apropoxylated bisphenol A fumarate polyester (ATLAC 382, sold by AtlasChemical Industries Inc.) and carbon black (5%) with similar results.

EXAMPLE XI A Xerox microprinter commercially available from XeroxCorporation in Stamford, Connecticut, is modified by the addition of 720DC power supply and the installation of a transfer corotron polarityswitch to provide for rapid switching from positive to negative modecopying and vice versa.

A developer is formed from the toner of Example I and a first carrier(electropositive) of 600 micron diameter flintshot coated with styrenemethyl methacrylate vinyl triethoxy silane polymer and a second carrier(electronegative) of 600 micron diameter flintshot coated with vinylchloride-vinyl acetate polymer.

The developer is used in various runs at electropositive toelectrone'gative carrier weight ratios which range from 1:1.23 to 1:1.44and toner concentrations which range from about 0.9 to about 1.2percent, by weight. Transfer is effected in both the positive mode(positive copy from positive original) and the negative mode (positivecopy from negative original at 10.5 amp.

The printer is operated in both the positive and negative mode andprints of satisfactory quality are obtained in both modes. The prints,in both the positive and negative mode, generally have an image densityof greater than 1.2, a background density of less than .03,

and a resolution of generally about 7 line pairs per millimeter.

EXAMPLE XII A developer carrier of 250 micron steel beads coated withpoly (vinylidene fluoride) is combined with a first toner comprising:

styrene-n-butyl methacrylate copolymer, polyvinyl butyral and carbonblack and a second toner comprised of styrene-n-butyl methacrylatecopolymer and carbon black, both produced as disclosed in Example I ofUS. Pat. No. 3,079,324 to Insalaco.

The developer comprised of a single carrier and two toners (the firsttoner acquires a positive charge and the second a negative charge) isused to develop an electrostatic latent image formed from a positiveoriginal on a flat selenium plate charged to +700 volts by cascading thedeveloper (three times) over the plate.

The image is transferred to paper using +700 volts and a positive copyof the positive original is produced.

The above is repeated, except that the image is transferred using 700volts, and a negative copy of the positive original is produced.

The above is repeated except that a negative original is used and upontransferwith +700 volts a negative copy of the negative original isproduced. Transfer effected with -700 volts produces a positive copy ofthe negative original.

The present invention is particularly advantageous in that a singlemachine is capable of producing either positive or negative prints fromeither positive or negative originals by selecting the charge ofpolarity placed on the toner receiving surface during transfer. In acommercial machine, the addition of a simple switch which alters thepolarity of charge placed on the paper during transfer provides thedesired flexibility.

The use of a single carrier and a single toner to provide bothnegatively and positively charged toner is particularly advantageous inthat there is no necessity to coordinate the amounts of two or moretoners, and- /or carriers as would be required in the case where morethan one carrier and/or toner is employed to provide both negative andpositive toner.

These and other advantages of the present invention should be apparentfrom the teachings herein.

Numerous modifications and variations of the present invention arepossible in light of the above teachings and, therefore, with the scopeof the appended 12 claims, the invention may be practiced otherwise thanas particularly described.

What is claimed is:

1. An electrostatographic developer, comprising: finely-divided tonerparticles of a single type and carrier therefor of a single type, thecarrier particles being larger than the toner particles, at least thesurface of the carrier being formed of at least two incompatiblematerials one of which is triboelectrically negative with respect tosaid toner and the other of which is triboelectrically positive withrespect to said toner, and said incompatible materials are capable ofimparting both a negative and a positive charge to said toner, wherebysaid toner electrostatically adhering to said carrier is both negativelyand positively charged on said carrier, the toner of one polaritycapable of being deposited in the image area during development and thetoner of the other polarity capable of being deposited in at least thebackground area immediately adjacent the image area during development.

2. The developer of claim 1 wherein said material capable of impartingboth a negative and a positive charge to said toner is an electret.

3. The developer of claim 2 wherein said electret is calcium titanate.

4. The developer of claim 1 wherein said at least two materials arepresent on the surface of said carrier in a domain size which is atleast about the average particle size of said toner.

5. The developer of claim 1 wherein said carrier is a coated carriersaid at least two materials forming the coating of said carrier.

6. The developer of claim 1 wherein said carrier is an uncoated carrierand said uncoated carrier is comprised of said at least two materials.

7. The developer of claim 6 wherein said carrier is comprised of bariumferrite dispersed in rubber.

8. The developer of claim 5 wherein said coating is comprised of calciumtitanate dispersed in selenium.

9. An electrostatographic developer, comprising: finely-divided tonerparticles of a single type and an uncoated carrier therefor of a singletype, said carrier comprising calcium titanate dispersed in selenium,whereby said toner electrostatically adhering to said carrier is bothnegatively and positively charged on said carrier.

1. AN ELECTROSTATOGRAPHIC DEVELOPER, COMPRISING: FINELYDIVIDED TONERPARTICLES OF A SINGLE TYPE AND A CARRIER THEREFOR OF A SINGLE TYPE, THECARRIER PARTICLES BEING LARGER THAN THE TONER PARTICLES, AT LEAST THESURFACE OF THE CARRIER BEING FORMED OF AT LEAST TWO INCOMPATIBLEMATERIALS ONE OF WHICH IS TRIBOELECTRICALLY NEGATIVE WITH RESPECT TOSAID TONER AND THE OTHER OF WHICH IS TRIBOELECTRICALLY POSITIVE WITHRESPECT TO SAID TONER, AND SAID INCOMPATIBLE MATERIALS ARE CAPABLE OFIMPARTING BOTH A NEGATIVE AND A POSITIVE CHARGE TO SAID TONER, WHEREBYSAID TONER ELECTROSTATICALLY ADHERING TO SAID CARRIER IS BOTH NEGATIVELYAND POSITIVELY CHARGED ON SAID CARRIER, THE TONER OF ONE POLARITYCAPABLE OF BEING DEPOSITED IN THE IMAGE AREA DURING DEVELOPMENT AND THETONER OF THE OTHER POLARITY CAPABLE OF BEING DEPOSITED IN AT LEAST THEBACKGROUND AREA IMMEDIATELY ADJACENT THE IMAGE AREA DURING DEVELOPMENT.2. The developer of claim 1 wherein said material capable of impartingboth a negative and a positive charge to said toner is an electret. 3.The developer of claim 2 wherein said electret is calcium titanate. 4.The developer of claim 1 wherein said at least two materials are presenton the surface of said carrier in a domain size which is at least aboutthe average particle size of said toner.
 5. The developer of claim 1wherein said carrier is a coated carrier said at least two materialsforming the coating of said carrier.
 6. The developer of claim 1 whereinsaid carrier is an uncoated carrier and said uncoated carrier iscomprised of said at least two materials.
 7. The developer of claim 6wherein said carrier is comprised Of barium ferrite dispersed in rubber.8. The developer of claim 5 wherein said coating is comprised of calciumtitanate dispersed in selenium.
 9. An electrostatographic developer,comprising: finely-divided toner particles of a single type and anuncoated carrier therefor of a single type, said carrier comprisingcalcium titanate dispersed in selenium, whereby said tonerelectrostatically adhering to said carrier is both negatively andpositively charged on said carrier.